Digitally controlled oscillators (DCOs) are commonly employed in a variety of applications, including communication and timing circuitry. In particular, DCOs are commonly used in phase-locked loop (PLL) systems. Functionally, a DCO may be viewed as a circuit that seeks to transform an input control voltage signal to an output voltage signal having a desired frequency.
In this case, the output signal of the PLL is normally compared against a reference signal, and a loop filter is used to tune the DCO in a manner dependent upon the comparison such that the output signal “matches” the reference signal. Such PLLs are usually used to synthesize signals at a desired frequency or, for example, to recover a clock signal from a data stream. PLLs can also be advantageously used in mobile radio for the purposes of signal modulation.